Summary:
useful when debugging to tell whether a DB has stats enabled, and whether a stats object is shared across DBs.
Closes https://github.com/facebook/rocksdb/pull/2813
Differential Revision: D5741755
Pulled By: ajkr
fbshipit-source-id: 9b9d51dee77d14d415cd5da985d8d61b5b3837c3